Low noise amplifiers (LNAs) are typically used in the front end stage of a radio frequency (RF) or microwave receiver, such as in a mobile telephone, to improve the receiver's sensitivity and ability to recover low power signals in the presence of noise. One of the most important parameters of a low noise amplifier is its noise figure, which should be kept as low as possible in order to successfully receive and recover signals at low power levels.
In some receivers, the received signal may have a wide range of input power levels. In that case, the low noise amplifier may be configured to selectively operate at different gain levels depending on the received signal level. That is, when the received signal level is low, the low noise amplifier may be controlled to operate in a high gain state with low noise, and when the received signal level is high, the low noise amplifier may operate in a reduced gain state where the noise figure is not as critical. In some low noise amplifiers, the amplifier may be selectively controlled or switched between more than two different gain levels or states, for example four, or eight, or more different gain levels. This is typically accomplished by changing the level of the bias current flowing through the low noise amplifier's transistor.
FIG. 1 illustrates an example of the output signal level of a low noise amplifier as the low noise amplifier is switched between various gain levels. As can be seen in FIG. 1, in each transition 110-1, 110-2, 110-3, 110-4 and 110-5, as the amplifier is switched from one gain state to another, the switching is not instantaneous but has a certain settling time before the amplifier output stabilizes to the new gain level.
Often there is a specification on the maximum allowable settling time for a low noise amplifier to switch between different gain levels or states. For example, in some receivers for mobile telephones, a maximum allowable settling time of 1 μsec. is specified for the low noise amplifier to settle to within 0.1 dB of its final output power level.
Typically, a dominant contributing factor to the setting time is an input RC time constant of the amplifier, which may be set by the parasitic input capacitance CP of the amplifier transistor and the impedance RB of the bias current source for the transistor. Thus to make the settling time faster, RB should be reduced. However, reducing RB also increases the noise figure for the low noise amplifier.
Thus there is a tension in conventional low noise amplifiers between achieving a fast settling time and providing a low noise figure.
What is needed, therefore, is a low noise amplifier which can achieve a fast settling time while providing a low noise figure.